Time-domain scan (TDS) is a method that performs “weighted” measurements across many frequencies during electromagnetic interference (EMI) emissions measurements. The weighted measurements are typically quasi-peak detector (QPD) measurements in which a measured signal level rises rapidly with a short rising time constant, but falls slowly with a longer decay time constant, thus emphasizing to a controlled degree a peak of the signal statistical distribution over its average value.
In a typical QPD measurement, the time constant for a signal to fall is around 0.5 s, and many time constants are required for the signal to settle. Consequently, it can take a long time to sweep through a frequency span. TDS, however, reduces this time burden by parallel processing. In particular, TDS uses fast Fourier transforms (FFTs) to generate an amplitude envelope at many frequencies from one time record. The amplitude envelope at each frequency is then assembled into a time waveform at each frequency which is then weighted with a weighting detector.
Conventional implementations of TDS suffer from various shortcomings that can limit the accuracy and/or cost of an EMI measurement system. For instance, in many conventional implementations of TDS, FFTs are performed with FFT windows that overlap by 90% or more in the time domain. This large overlap means that a relatively large number of FFTs must be performed, which in turn means that the measurement system must include more processing capability, e.g., extra hardware. On the other hand, smaller overlap tends to produce so-called time scalloping errors, which correspond to undesired variation in a computed spectrum with a time relationship between the FFT starting times and the signal event.